You receive extra Swaps according to challenge participation, see more info here.
You can also purchase more Swaps at any time you'd like.
Simply press the 'Swap' button at the top left corner or underneath your photos in a challenge.
This opens the store where you can choose the package you want.
Proceed by filling all the required fields and pressing the submit button.
The Swaps will be added to your total and a receipt sent to the mail you provided.